The study shall evaluate the efficacy and safety of two different hydroxyethyl starch solutions (artificial colloids 6% HES130/0.4 and 6% HES70/0.5, Salinhes®) for intra-operative therapy of hypovolemia and maintenance of circulating blood volume in patients undergoing orthopedic surgery. The primary efficacy endpoint will be the required volume of colloid solution infused from start until end of surgery. It is the aim of the clinical trial to demonstrate that comparable volumes of colloid solution are used between treatment groups.